Die-cuts of various adhesive materials, as in adhesive tapes and bonding films, are used to mechanically assemble various components or parts together. In these instances, adhesive die-cuts are typically placed between two (2) parts to join them together. In its simplest form, appropriate pressure for a recommended period of time is applied to two parts stacked on top of each other with a pressure sensitive adhesive (PSA) die-cut sandwiched between them for assembly. In general, PSA tape die-cuts need moderate pressure in the range of about 15 psi for a few seconds to bond parts at normal room temperature.
For certain assembly applications, thermal compression bonding is used to achieve both improved physical contact between the parts and the PSA die-cut used to hold the parts together as well as higher initial bonding strength. Adhesive bonding film die-cuts, either thermoset or thermoplastic, typically require heat and pressure for a period of time to achieve bonding. For thermoset materials, the heat chemically cures the adhesive. For thermoplastic materials, the heat softens the material for proper wetting onto the surfaces of the parts to be assembled. The temperature range for heating the bonding films depends on the material choice and is typically about 70° C. to 200° C.
Adhesive die-cuts are usually manufactured by cutting chosen adhesive materials into desired geometric patterns. Typically, for organic materials such as films, adhesive tapes, pads, foams and others, mechanical stamping dies are used to manufacture the die-cuts. Computer controlled cutting tools, such as knife and laser cutters, could also be used. In addition, other means can be employed to manufacture die-cuts as well.
Thermal compression bonding machines 30 (see FIGS. 1A-1D) used in the industry today have common fundamental components: pneumatic cylinders 32, heating blocks 34, top fixtures 36 that are part of heating blocks 34 or are heated by heating blocks 34, bottom fixtures that periodically are brought into a pressing or abutting engagement with the top fixtures 36, electronic controllers, and timers. Some machines 30 also have conveyors for moving the parts into and out of the machine 30. FIGS. 1A and 1B show a thermal compression bonding machine 30 having two work stations that are generally identical to one another, namely a left work station 38I and a right work station 38r. FIGS. 1C and 1D show a single work station inside a thermal compression bonding machine 30. Obviously, compression bonding machine 30 may have different numbers of work stations 38.
The top fixtures 36 and their mating bottom fixtures are typically CNC machined according to the 3D geometry of the parts to be assembled. The fixtures secure the parts and prevent them from shifting during the thermal compression bonding process and while conveying the parts inside the machine. The pneumatic cylinders 30 are used to apply force to the stacked parts. The heating blocks 34 and timers are used to deliver heat for a set duration of time to the stacked parts to achieve a certain peak temperature range. In some cases as shown in FIGS. 1C and 1D, a cooling station having a cooling block 39 that cools a top fixture 36 is incorporated in the machine 30 to cool the assembled parts after the heating cycle.
In a typical thermal compression bonding process, two (2) parts 40 and 42 with adhesive (not shown) sandwiched between them (see FIG. 2) are loaded into the machine 30. By way of example but not limitation, the parts 40 and 42 shown in FIG. 2 are various parts of a case or housing of a cell phone. The bottom part 40 is laid atop or nested onto the bottom fixture 16 as shown in FIG. 2C. The top part 42 is then laid on top of the bottom part 40 with adhesive sandwiched between the top and bottom parts, the adhesive typically being in the form of a die-cut whose geometry mimics the contact surfaces of the top and bottom parts 40 and 42. Since the bottom part 40 is nested onto the bottom fixture 16, this nesting secures the entire assembly of the bottom part 40, sandwiched adhesive (not shown), and top part 42 to the bottom fixture 16 during conveying inside the machine 30 and prevents shifting of the assembly on the bottom fixture 16 while compression force is applied thereto by top fixture 36.
The machine 30 is programmed with a set temperature for the heating blocks 34 and time duration for heating. The top fixture 36 that conforms to the 3D surface geometry of the top part 42 is mounted to the heating block 34 which is attached to the piston of the pneumatic cylinder 32. The piston 32/heating block 34/top fixture 36 assembly travels downward to a set height and applies force to the stacked parts 40 and 42 by compressing the parts 40 and 42 between the top and bottom fixtures 36 and 16. For a cooling cycle, cooling air may be channeled into the top fixture 36 and/or bottom fixtures 16 to cool the assembled parts after the heating cycle. In some machines, simple spring mechanisms or conformable pads are used to apply cushioning during the compression.
All production parts, such as the parts 40 and 42, have geometric tolerances as they are mass manufactured. These part tolerances largely determine the required minimum thickness of the adhesive, selection of the adhesive, assembly yield and reliability of the adhesive assembled parts. As a rule of thumb, the minimum adhesive thickness is twice the combined tolerance of the parts. As an example, if part A has a coplanarity tolerance of 50 microns and part B has a coplanarity tolerance of 100 microns, the minimum adhesive thickness would be 300 microns. The idea behind this general guideline is to have enough adhesive material to fill the gap created by the stacking tolerance of the two (2) parts to have cohesive strength of the adhesive.
For assembly of high volume production parts using thermal compression adhesive bonding, the production yield, the strength and the quality of the adhesive bonding depend on the geometric tolerance of the parts. The adhesive must be in intimate contact with the surfaces of the parts to physically bond the parts. Gaps exist between the surfaces of production parts because of geometric tolerances that are inherent during the manufacture of the parts themselves. Across the contact surfaces of parts, if there are numerous locations with gaps larger than the thickness of the adhesive, the bonding performance would be lower than optimum and the quality would be inconsistent. For gaps that are substantially larger than the thickness of the adhesive and located at critical locations, the bonding assembly yield would be low because of incomplete adhesive bonding not satisfying the minimum bonding strength requirement.
As noted earlier above, in thermal compression bonding of adhesives, the typical equipment used in the industry has means to heat the adhesive via conduction and apply compressive force to the stacked parts. In most cases, pneumatic cylinders 32 are used to apply the force (see FIGS. 1A-1D). Other means of applying force are via motors and combinations of motors and pneumatic cylinders. For proper adhesive bonding, the temperature, the time, and the pressure ranges need to be controlled for the chosen adhesive. For thermal compression bonding of thermoplastic adhesive bonding films, the softening temperature of the film need to be achieved in the heating stage using proper pressure and the adhesive needs to be cooled under proper compression to achieve adhesive bonding.
Current thermal compression adhesive bonding machines in the industry control the distance the piston of the pneumatic cylinder travels by adjusting the stopping position of the piston in relation to the top of the stacked parts. In such cases, the stopping position of the cylinder piston is adjusted to control the push-down distance of the stacked parts. This distance is equal to the partial thickness of the adhesive and is difficult to control across the tolerance range of the parts. This method exerts a force on the parts that is equal to the force exerted by the compressed air onto the area of the piston. This force can be much greater than the desired force needed for thermal compression bonding.
In addition, the Z-direction geometric tolerances of the stacked production parts affect the relative distance between the top of the stacked parts and the bottom of the piston in its extended state. Depending on the tolerances of the stacked parts, the heating and the cooling fixtures attached to the bottom of the cylinder may not make contact with the top of the stacked parts. Pressure control over the tolerance range of the production parts is difficult. This may result in a longer time to set up the machine using a trial and error method of testing. As new production tools are added for the increase in mass production of parts, the machine needs to be set up again for the changes in the tolerance range of parts.
Some machines use springs or elastomeric materials to cushion the piston force at the moment of contact with the stacked parts. Some machines in the industry use compression of springs as the only mechanism of distributing the pressure. Another simple means of controlling the force and its resultant pressure is adjusting the input air pressure to the cylinder and using the areas of the piston and common contact area of the parts to be assembled. The shortcoming of this method is that the force exerted by the piston varies as the input air pressure fluctuates.